1. Field of the Invention
The present invention relates to a rotor structure of an electrical motor, and more particularly, to a rotor structure of the type in which permanent magnets are inserted.
2. Description of the Related Art
Various types of motors are being developed because motors are essential for obtaining revolving power. Nevertheless, motors occupy a large portion of the total volume and weight of the electronic devices in which they are incorporated. Thus, the R&D trend is formed for developing small sized and lightweight motors for achieving small sized and lightweight electronic devices.
An AC motor adaptable for various electric home appliances obtains revolving power by producing a revolving magnetic field from an AC voltage supplied to a stator. Reducing the size of an AC motor is difficult because of various electric losses occurring during electric current flow in a rotor, and difficulty in performing a coil winding process on the core of a motor rotor.
Furthermore, demand for a DC motor is rapidly growing in devices requiring even torque characteristics, so, R&D in connection with DC motors is progressing. However, the life span of a DC motor is short because a DC motor basically requires a commutator that produces mechanical losses and generates friction between the commutator and a rotor shaft.
Accordingly, R&D is being performed for a motor that obtains torque without a commutator. A brushless and a switched reluctance motor are representative results of that research. The brushless motor uses an electronic method as a substitute for a commutator through turning electric current and magnetic field direction, subsequently, and installing a permanent magnet on a rotor. The switched reluctance motor in which rotor revolution depends on a reluctance change of the magnetic field is developed by supplying each phase of AC electric current to a stator, installing a permanent magnet on a rotor and switching current flowing through each phase of winding coils on a stator and a permanent magnet of a rotor.
The rotors for such a brushless and a switched reluctance motor need to be operated without electric losses and vibration at the high rotary speeds at which such motors operate.
Permanent magnet type rotors used for general purposes are divided into a permanent magnet exterior type rotor and a permanent magnet inserted type rotor.
FIG. 1 is a perspective view illustrating a permanent magnet exterior type rotor of a conventional motor.
FIG. 2 is a cross-sectional view illustrating a permanent magnet inserted type rotor of a conventional motor.
As shown in FIG. 1, a permanent magnet exterior type rotor is composed of a rotor core 3 formed by stacking steel rotor plates 2 made of thin silicon steel. Permanent magnets 5, each formed as a segment of a cylinder, are bonded to the outer peripheral surface of the core by adhesive 4. A hole 1 is formed in the central part of the core for receiving a rotor shaft (not shown) by press fit.
In such a permanent magnet exterior type rotor, the adhesive may lose adhesive power according to long term use, whereby a permanent magnet 5 may become separated from the core 3. In addition, at a high rotary speed of the motor, a permanent magnet 5 may separate from the rotor core 3 due to the centrifugal force being produced.
Consequently, a permanent magnet inserted type rotor as shown in FIG. 2 is usually used for high speed applications.
As shown in FIG. 2, in the permanent magnet inserted type rotor, straight magnet receiving slots 21 of predetermined length are symmetrically formed about the center of steel plate 2 perpendicular to a radial direction.
When the rotor core 3 is formed by stacking steel rotor plates 2, the straight magnet receiving slots 21 are formed in the rotor core 3 in the longitudinal direction. As shown in FIG. 3, a magnetic field is produced by permanent magnets 7 disposed in the straight magnet receiving slots 21.
However, in such a rotor structure, a rotor may become damaged by high levels of stress occurring at the ends 22 of each straight magnet receiving slot 21. That stress is greater than the stress occurring elsewhere in the rotor, producing an uneven stress distribution that may cause vibration and breakage of the rotor at a high rotary speed.
To solve the above-mentioned problem, much R&D has been performed, and one result of such efforts is described in Japanese Patent Application Disclosure Gazette No. 5-236,685, disclosed on Oct. 9, 1993.
In that patent application, an even stress distribution can be assured by straight permanent magnet receiving slots 21 having ends extending generally radially for a predetermined length, as shown in FIG. 4.
As shown in FIG. 4, in the steel rotor plate 2 of the permanent magnet inserted type rotor 6, straight magnet receiving slots 21 of a predetermined length are formed perpendicular to the radial direction, and have their ends are connected to extended slots 42 of predetermined length in the radial direction. Each radial slot 42 is parallel to another radial slot 42. An even stress distribution obtained by the presence of the radial slots 42 in the steel rotor plate 2 prevents motor vibration.
The presence of the radially extended slots 42 makes stress distribution even, but the energy is inefficiently consumed because a disturbing area 43, formed perpendicularly to the path of magnetic flux between adjacent parallel radial slots 42, may cause magnetic flux losses, as explained below.
FIG. 5 shows a magnetic flux distribution in a conventional motor. The magnetic flux, excited by electrification of winding coils on the stator 51 and traveling between the stator 51 and the rotor 6 through air gaps formed between the rotor 6 and the stator 51, is disturbed in the disturbing areas 43 disposed between the pairs of adjacent extended slots 42 of a rotor 6, so the magnetic flux path is distorted and magnetic resistance is increased. Furthermore, there occurs much magnetic flux leakage, i.e., magnetic flux produced by the permanent magnets of the rotor 6 does not flow to the air gap but rather circulates within the rotor 6.
FIG. 6 shows a torque measurement graph for the rotor shown in FIG. 2. As shown in FIG. 6, with reference to the mechanical angle in the direction of the motor revolution, the difference between the maximum torque value and the minimum torque value is large, and the ripple between the maximum torque value and the minimum torque value that may cause vibration is rapidly formed. Consequently, mechanical vibration and noises are produced. Also, the average torque per unit stacking of the rotor steel plate 2 is about 3 kg-cm.